Fundamental Study of Mixed-Phase Icing with Application to Ice Crystal Accretion in Aircraft Jet Engines

Abstract

This paper describes experiments performed in an altitude chamber at the NationalResearch Council of Canada (NRC) as the first phase of a joint NRC/NASA program investigating ice crystal accretion in aero engines. The principal objective was to explore the effect of wet bulb temperature T wb (dependent on air temperature, humidity and pressure) on accretion behavior, since preliminary results published in an earlier paper indicated that well-adhered accretions are only possible at T wb <0°C, when water in an impinging mixedphase flow can freeze to a surface. To assess the accretion sensitivity to T wb , the symmetrical airfoil used in the previous work was tested at pressures of 44.8 kPa and 93kPa, usually at 0.25 Mach number, over a range of freestream liquid water and ice water concentrations, total air temperatures and humidity levels. T wb was typically maintained at +2°C or -2°C, based on dry total conditions (i.e. without ice or water injection). Total air temperature was >0°C in all tests. The limited test results confirmed that accretion behavior is very sensitive to T wb , which is in turn strongly related to pressure since evaporative cooling increases with decreasing pressure. Humidity and total temperature did not appear to have an independent effect on accretion behavior. Accretions, often resembling glaze ice, formed at T wb <0°C, when freestream water would freeze on the test airfoil without ice crystals present in the freestream. At T wb >0°C ice deposits were observed to be slushy, poorly adhered and shed frequently. The size of such deposits appeared to be a non-linear function of the freestream ice water content (IWC), becoming much larger at high IWC. NomenclatureAOA = airfoil angle of attack (°), positive for nose up C p = specific heat D = diffusion coefficient of water vapor in air h c , h m = heat, mass transfer coefficient k = thermal conductivity 1 2 L evap = latent heat of vaporization of water Le = Lewis number (= Pr/Sc) LE = wedge airfoil leading edge IW LW = mass flux C = Ice Water Content (g/m 3 ) C = Liquid Water Content (g/m 3 ) M = Mach number M wt = molecular weight MMD = median mass diameter of ice particles, micrometers NASA = National Aeronautics and Space Administration NRC = National Research Council of Canada Nu = Nusselt number (= h c x/k) p = pressure P P = heat flux a = Pascal (N/m 2 ) r = Prandtl number (= µC p /k) RATFac = Research Altitude Test Facility Re = Reynolds number (= ρUx/µ) RH = relative humidity, % R u = universal gas constant Sc = Schmidt number (= µ/ρD) SH = specific humidity (mass of water vapor/unit mass dry air) t = ice thickness T = temperature TWC = Total Water Content (= IWC + LWC) U = velocity x = distance (length) µ = dynamic viscosity ρ = density ρ acc = accretion density (≈( IWC* ρ ice + LWC* ρ liquid )/TWC, for ice and liquid densities ρ ice and ρ liquid respectively) Subscripts conv = convective evap = evaporative i = injected with the ice grinder or water spray nozzles. Ice or water mass flowrate divided by tunnel air volumetric flowrate to ...

Eulerian method for ice crystal icing in turbofan engines

“…This is confirmed by experiments that have been conducted by Currie et al [23]. In these experiments the effect of the wet bulb temperature on the accretion behaviour has been studied in the altitude test facility of NRC (RATFac).…”

“…• C. From these experiments, and those on the double-wedge aerofoil (Currie et al [23]), it has been concluded that erosion of ice accretions mainly depends on the tangential velocity of the impinging ice crystals and that it scales with that velocity to a power 2 or 3. This conclusion has been based on the observation that adding ice crystals to a free stream with a wet bulb temperature below freezing led to an increase of ice growth on the leading edge, but to a decrease of ice growth further downstream.…”

“…• erosion scales with the square of the tangential velocity [23,24,25], • erosion decreases for increasing IWC due to flux interference [24,25], • erosion increases for increasing melting ratio η i c = LWC i c /TWC [24,25], • erosion increases with increasing ice crystal diameter d i c [71,72], • erosion increases with increasing curvature of the surface of the ice layer [132].…”

“…The first three of these facilities are capable of simulating ice crystals under various altitude conditions. These upgrades have resulted into a number of experimental studies on glaciated and mixed-phase accretion for various geometries: an aerofoil shape by Struk et al [127], a NACA-0012 aerofoil by Al-Khalil et al [2] and Struk et al [126], a double-wedge aerofoil by Currie et al [23,22] and Struk et al [124,126], a streamlined body with an axi-symmetric hemispherical nose, with a cylindrical nose and with a conical nose by Currie et al [22,24,25], a bleed slot in a compressor stage by Knezevici et al [71,72] and an aerofoil in a compressor transition S-duct by Mason et al [96].…”

“…Parameter K B has a value of 40 which is obtained from an empirical fit to experimental data from Currie et al [23] obtained for leading edge ice accretion. The disadvantage of the relation given in Equation (4.3) is that the height of the liquid film is required, which is difficult to estimate.…”